Asymmetric monometallic nanorod nanoparticle dimer and related compositions and methods

Abstract

The fabrication of asymmetric monometallic nanocrystals with novel properties for plasmonics, nanophotonics and nanoelectronics. Asymmetric monometallic plasmonic nanocrystals are of both fundamental synthetic challenge and practical significance. In an example, a thiol-ligand mediated growth strategy that enables the synthesis of unprecedented Au Nanorod-Au Nanoparticle (AuNR-AuNP) dimers from pre-synthesized AuNR seeds. Using high-resolution electron microscopy and tomography, crystal structure and three-dimensional morphology of the dimer, as well as the growth pathway of the AuNP on the AuNR seed, was investigated for this example. The dimer exhibits an extraordinary broadband optical extinction spectrum spanning the UV, visible, and near infrared regions (300-1300 nm). This unexpected property makes the AuNR-AuNP dimer example useful for many nanophotonic applications. In two experiments, the dimer example was tested as a surface-enhanced Raman scattering (SERS) substrate and a solar light harvester for photothermal conversion, in comparison with the mixture of AuNR and AuNP. In the SERS experiment, the dimer example showed an enhancement factor about 10 times higher than that of the mixture, when the excitation wavelength (660 nm) was off the two surface plasmon resonance (SPR) bands of the mixture. In the photothermal conversion experiment under simulated sunlight illumination, the dimer example exhibited an energy conversion efficiency about 1.4 times as high as that of the mixture.

Description

CROSS REFERENCE TO RELATED APPLICATION[0001]This is a national stage of PCT / IB2015 / 002077, filed internationally on Sep. 25, 2015, which claims priority to U.S. Provisional Application Ser. No. 62 / 055,369, filed on Sep. 25, 2014, both of which are incorporated by reference herein in their entireties.INTRODUCTION[0002]Colloidal noble metal nanocrystals (NCs) usually exhibit highly symmetric particle morphologies, as dictated by their intrinsic crystallographic symmetries. Breaking such morphological symmetry would bring vast variations in their plasmonic and chemical properties, greatly enriching their applications in plasmonics,1 nanophotonics,2 sensing3 and surface enhanced Raman scattering (SERS).4 [0003]One effective approach to symmetry breaking is to synthesize “dimer” crystals via seeded growth, where one can vary many factors, such as the size and shape of the seeds, the type of the growth material, the choice of capping ligand, and the reaction kinetics, to manipulate the di...

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Benefits of technology

[0004]It is interesting to note that in addition to being a fundamental challenge in wet-chemistry, fabricating such an asymmetric monometallic dimer structure has significant implications in nanophotonics. Theoretical calculation based on Conformal Mapping Transformation8 indicates that a dimeric NC consisting of a spherical AuNP grown on a AuNR might behave as a “black” material, with high absorption in an extremely broad spectral window ranging from near ultraviolet (UV) to far infrared (IR). This is in significant contrast to conventional plasmonic NCs, which inherently show narrow-band plasmon resonances absorption. This unique property endows the proposed AuNR-AuNP dimer with a great promise as a black super absorber to enhance the performance of photovoltaiccells, blackbody emitters, and plasmonic lasers. It would also be useful as a multiple-frequency applicable platform for photothermal conversion and field enhancement spectroscopy.

[0014]Herein, it is described in a preferred embodiment that with properly chosen experimental variables such as ligand selection, reducing agent, and reaction conditions, asymmetric AuNR-AuNP dimers can be synthesized from single crystalline AuNR seeds. These dimers show extraordinary broadband optical extinction behavior spanning from UV to near IR, a distinguishing feature from AuNRs, AuNPs or their mixture. Using electron microscopy and tomography techniques, the inventor(s) reconstructed the three-dimensional morphology of the dimer crystal, and identified that the newly grown AuNP is a multiple twinned crystal preferentially residing at the “neck” region of the AuNR. The well-defined morphology of single crystalline AuNR (with surface facets' indices known) allows one to easily identify the formation of dimer interface and to track the subsequent evolution of AuNP. While the claimed inventions are not limited by theory, the results provide important insights into the growth mechanism of this unprecedented dimeric nanostructure, suggesting that the NP first nucleates at one (111) bridging facet of the NR, which further grows as a consequence of random twinning, and finally re-crystallizes into a single multi-twinned crystal.

[0016]In a preferred embodiment, asymmetric monometallic Au dimer nanocrystals have been synthesized by site-specific growth of single AuNPs on single-crystalline AuNRs. The growth pathway has been explicitly identified by detailed TEM study. Thiol-based ligand such as, for example, components such as 4-MP can play important roles in directing the formation of the dimer structure. Thermodynamically, it induces significant and inhomogeneous surface strain on the AuNR to initialize the growth of AuNP from stacking faults; kinetically, it modulates the reduction rate of the Au precursor to prevent uncontrolled deposition. The obtained AuNR-AuNP dimer exhibits a unique broadband optical extinction behavior, which is substantially distinct from the SPR-based extinction by AuNP, AuNR, and their physical mixture. The inventor(s)′ results demonstrate, for example, that the broadband extinction property makes the dimer a generally applicable SERS substrate for various excitation wavelengths and an effective photothermal convertor using solar light.

Problems solved by technology

Given the high degree of symmetry of metallic structures, it is conceivable that growing a single NP on a highly regular NR is difficult because it requires one site on the NR at which the NP is grown to be differentiated from many other symmetry equivalent sites.

For example, the use of thiol-based ligands that strongly interact with the Au seeds could induce asymmetric growth of Ag, which would otherwise form a concentric shell over the seeds due to the little lattice mismatch between Ag and Au.6a However, the mechanism of such ligand-induced asymmetric growth remains subject of review due to the lack of direct experimental evidence, and to the best of inventor(s)′ knowledge this strategy has not been applied to grow dimers from single crystalline NCs with regular shapes.

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